A most typical method of detecting whether a radio signal is present is envelop detection. In a typical method of implementing the envelop detection, an input signal is squared or calculates the absolute value of the input signal, and then filtered by a low pass filter. When the time constant of the low pass filter is sufficiently large, most of the high-frequency component of the input signal can be removed and an output signal that has been smoothed can be acquired. Therefore, the signal that has been smoothed and an arbitrary reference signal are compared in intensity so that whether the signal is present can be determined.
However, when the amplitude of the input signal considerably varies, the envelop detection is performed with the low pass filter having a time constant sufficiently large so that the output signal level of the low pass filter decreases and the detection accuracy of a reception signal degrades.